make.conf and CCVER

Greetings,
If I set the CCVER variable in the make.conf (uncommenting either of the
two example lines) I get the following error whenever I try to compile a
port:
"/usr/share/mk/bsd.cpu.mk", line 30: "Either set CCVER to a known compiler or sp
ecify CCVER_BSD_CPU_MK"
Looks like the variable is somehow being unset.
Marco